My question is, how everyone get the exact dielectric constant that they want at a certain frequency after using the same dielectric permittivity formulas, I mean the perturbation is slight so that doesn't change too much, the volume of cavity is also same, and if the the substrate is common and thin film is few nm, so how can one get the exact dielectric constant value, is it the thin film thickness that is really low??? Any help is appreciated. Thank you